Why Do Stars Twinkle When Planets Shine Steadily?

Lie on a blanket in the garden on a clear summer night and the sky seems alive. Stars sparkle, flicker and shimmer as if someone were gently shaking a jar of glitter overhead. Then you notice one bright point that does not join in. It just glows, calm and steady, while everything around it dances. There is a good chance you have found a planet.

The old trick for telling them apart really works most of the time: stars twinkle, planets usually do not. But the reason is not that stars are nervous and planets are relaxed. Neither of them is flickering at all. The twinkle is made much closer to home, in the very last stretch of the light's long journey.

The short answer

Starlight crosses the emptiness of space in perfectly straight lines, and then it meets our atmosphere. The air above us is a restless patchwork of warmer and cooler pockets, and they bend light by slightly different amounts. The Hong Kong Observatory explains that warm air bends light less than cool air, so a ray of starlight passing through air of different temperatures is nudged one way and then another. Astronomers call the effect scintillation. A star is so far away that it reaches us as a single point of light, so every nudge shows: the star seems to jump, brighten, fade and briefly vanish.

Why a planet shines steadily

A planet is far closer to us than any star in the night sky, so even though it looks like a dot, it is really a tiny disc. EarthSky's explanation puts it neatly: light from one edge of the disc may zig one way while light from the other edge zags the other, and the zigs and zags cancel out. The planet is made of many points of light at once, and their little wobbles average into a steady glow.

What people often get wrong

The first mistake is thinking that the twinkle belongs to the star. Seen from above the atmosphere, stars shine perfectly steadily. The second is thinking that planets never twinkle. Low in the sky, near the horizon, you look through far more air than when you look straight up, and there even a planet can flicker. That is also why stars near the horizon twinkle more than the ones overhead.

How astronomers undo the twinkle

Twinkling is lovely for anyone on a blanket, but it blurs the view for astronomers, who want every star to be a sharp, clean point. At the European Southern Observatory's Very Large Telescope in Chile, a laser shines up into the sky and makes sodium atoms about 90 kilometres above the ground glow. The result is a bright artificial star.

The telescope watches how that artificial star twinkles and feeds the measurements to a mirror that can change its shape, constantly correcting the picture so that stars become sharp points again. The trick is called adaptive optics, and ESO describes the result as almost as though the telescope were above the atmosphere in space. So the next time a star winks at you, it is really the air winking, and somewhere on a mountain, a mirror is quietly winking back.